1. Field of the Invention
The present invention relates to an apparatus for supplying crystalline materials which is to be used in growing single crystals by the Czochralski method, and to a method of supplying crystalline materials which is to be employed in that apparatus. More particularly, it relates to an apparatus for supplying crystalline materials which is to be used for additional charging or recharging of a solid material to the molten material in a crucible in growing a silicon single crystal, and to a method of supplying crystalline materials which is to be employed in that apparatus.
2. Description of the Related Art
Generally, in growing silicon single crystals by the Czochralski method, the solid single crystalline silicon charged into a crucible as the initial charge is molten upon heating by a heater surrounding the crucible. After formation of a molten material in the crucible, a seed crystal held above the crucible is lowered until it is inserted into the molten material in the crucible, while the crucible is rotated in a fixed direction. The seed crystal is then pulled up while it is rotated in a given direction. A cylindrical silicon single crystal is thus grown following the seed crystal.
Used as the solid material to be charged into the crucible as the initial charge is polycrystalline silicon in various forms, such as rod-like, lump-like or granular forms. These forms are supplied singly or in combination and serve as the materials constituting the molten material for silicon single crystal growing.
In silicon single crystal growing by the Czochralski method, the melting of the solid material initially charged in the crucible results in a decrease in volume, so that the volume of the resulting molten material becomes too small as compared with the capacity of the crucible. If single crystal growing is carried out in such a condition, a decrease in productivity is unavoidable due to the insufficient quantity of the molten material.
For avoiding the decrease in productivity resulting from the above, it is necessary to secure a desired amount of the molten material by supplementing the deficiency in molten material. Thus, “additional charging” is carried out as a technique for additionally supplying the solid material after initial charging into the crucible.
Thus, this “additional charging” is a technique for increasing the amount of the molten material in the crucible by further adding the solid material to the molten material after melting of the solid material initially charged in the crucible. By applying this “additional charging”, it becomes possible to efficiently utilize the capacity of the crucible used and thereby improve the productivity in silicon single crystal growing.
Further, in growing silicon single crystals by the Czochralski method, a solid material supplying technique called “recharging” is also employed. More specifically, this is a technique adding, to the molten material remaining in the crucible after growing and pulling up a first single crystal, the solid material in an amount corresponding to the decrease in an amount of the molten material.
In other words, this technique consists in increasing the number of crystals pulled up per crucible by reforming a given amount of the molten material in the crucible and repeating the pulling up of a single crystal.
Therefore, by employing “recharging”, it becomes possible to attain cost reduction through efficient utilization of the crucible and to improve the productivity and thereby reduce the cost of growing silicon single crystals, like in the case of “additional charging” mentioned above.
However, the material supply by additional charging or recharging is made in the art by adding the solid material in granular form to the molten material in the crucible using a material feeding pipe inserted in the growing furnace. Therefore, problems arise; for example, the solid material additionally charged may damage the crucible or cause splashing of the molten material, or splashes of the molten material may adhere to the parts in the crucible and shorten the life thereof or adversely affect the process of single crystal growing.
Therefore, various proposals concerning additional charging or recharging have been made in the art. For example, in view of the fact that the quartz crucible is readily damaged on the occasion of recharging and this may readily cause dislocation during single crystal growing, Japanese Patent Application Laid-open No. 09-208368 proposes a method of supplying silicon materials by which silicon materials can be additionally supplied in a manner friendly to both the silicon melt surface and solidified surface, rapidly and without damaging the quartz crucible on the occasion of melting.
It is alleged that by the method proposed in the above-cited publication, the splashing of the molten material can be avoided and the productivity in silicon single crystal growing and the production yield can be improved.
Furthermore, since direct addition of the solid material to the material in the crucible causes splashing of the melt, a technology is employed which comprises allowing the surface of the melt remaining in the crucible after withdrawal of the initial single crystal to solidify to a certain extent, feeding the solid material onto the solidified surface, and then melting the same. In this case, it is necessary for the operator to visually observe the state of solidification of the melt surface. During such observation, the operator cannot conduct any other work, which will be a hindrance to productivity improvement.
Therefore, for lightening such restriction on operator's action and making it possible to strive for improvement in productivity, Japanese Patent Application Laid-open No. 11-236290 proposes an additional material charging system for use in single crystal pulling apparatus in which system the state of solidification of the melt surface is detected by means of a visual sensor. This additional material charging system will make it possible to lighten operator's work and allow the operator to conduct some other work and thereby improve the productivity.